Fundamentals of a floating loop concept based on R134a refrigerant cooling of high heat flux electronics

The Oak Ridge National Laboratory (ORNL) Power Electronics and Electric Machinery Research Center (PEEMRC) has been developing technologies to address the thermal concerns associated with hybrid electric vehicles (HEVs). This work is part of the ongoing FreedomCAR and Vehicle Technologies program (FCVT), performed for the Department of Energy (DOE). Removal of the heat generated from electrical losses in traction motors and their associated power electronics is essential for the reliable operation of motors and power electronics. As part of a larger thermal management project, which includes shrinking inverter size and direct cooling of electronics, ORNL has developed U.S. Patent No. 6,772,603 B2, Methods and Apparatus for Thermal Management of Vehicle Systems and Components (Hsu, 2004), and patent pending floating loop system for cooling integrated motors and inverters using hot liquid refrigerant (Hsu, 2004). The floating-loop system provides a large coefficient of performance (COP) for hybrid-electric drive component cooling. This loop (based on R-134a) shares a vehicle's existing air-conditioning (AC) condenser, which dissipates waste heat to the ambient air. Because the temperature requirements for cooling of power electronics and electric machines are not as low as that required for passenger compartment air, this adjoining loop can operate on the high-pressure side of the existing AC system. This arrangement also allows for the floating loop to run without the need for the compressor and only needs a small pump to move the liquid refrigerant. For the design to be viable, the loop must not adversely affect the existing system. The loop would also provide a high COP, a flat temperature profile, and a low pressure drop. The floating-loop test prototype has been successfully integrated into a 9 kW automobile passenger AC system. In this configuration, the floating loop has been tested up to 2 kW of heat rejected during operation with and without the automotive AC system running. The floating-loop system has demonstrated a very respectable COP of 40-45, as compared to a typical AC system COP of about 2-4. The estimated required waste-heat load for future HEV cooling applications is 5.5 kW, and the existing system should be easily scalable to this larger load